1. Field of the Invention
The present invention relates to a frame structure of a motorcycle, and more particularly, to a new structure of a pivot plate supporting a swing arm.
2. Description of the Background Art
Japanese Utility Model Laid-Open No. H1-21919 discloses a vehicle frame structure in which the inserting portion of the pipe member is formed by jointing two plate members into an oval shape, and the pipe member is inserted therein and welded in one body. Furthermore, it is a known practice to form the pivot frame which supports the swing arm by forging.
The pivot plates of the aforesaid construction must be attached to a frame member by welding, and therefore cannot be easily positioned properly at this time. Furthermore, in the case of an off-road motorcycle which undergoes a great impact load during operation, the frame structure is required to have a greater rigidity. To realize this in the prior art frame structure, it is necessary to considerably increase the weight. It is, therefore, an object of this invention to provide an easy-to-assemble, light-weight frame structure capable of obtaining a great rigidity.
To solve the above-described problem, the frame structure of a motorcycle according to this invention has one main pipe extended from a head pipe to the rear, and passing above an engine. A pivot frame is attached at the upper end side to the rear end of the main pipe and extends downwardly to support the front end of a swing arm capable of swinging movements. The frame structure of a motorcycle is characterized in that the pivot frame is provided with a pair of right and left pivot plates which pivotally support the swing arm, and an upper cross member attaching the upper end of the pivot plates. The upper cross member is provided with a cross section extended laterally, attaching the upper end of the pivot plates to the right and left ends. The pivot plates are formed by forging.
The frame structure has an upper cross member, the cross member of which project laterally at either end. This enables the right and left pivot plates overlapped with the forward end of each cross member to be attached by welding. Forming a shallow recess in the pivot plate jointing section is sufficient, and therefore a high-rigidity forging is usable, consequently enabling weight reduction despite its high rigidity.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.